Fluoaluminate composition as dielectric material

ABSTRACT

A ternary solid solution of potassium fluoaluminate, rubidium fluoaluminate and potassium fluoborate preferably containing 1-15 moles per cent, more preferably 1-5, of KBF4, 4-90 moles per cent of RbAlF4 and the remaining proportion KAlF4 is prepared by cooling a melt of the constituents in the required composition. Proportions of 2-5% KBF4 and 4-50% RbAlF4 are preferred from a practical standpoint. The KBF4 may be prepared by the method of Borlander, Hollatz and Fisher, i.e. the reaction of H3BO3 and HF in the presence of KOH. KALF4 and RbAlF4 are prepared by Brosser&#39;s method, i.e. evaporating to dryness a mixture of (1) an aqueous HF solution of hydrated alumina and (2) K or Rb fluoride, the solid ingredients being in stoichiometric quantity.  In an example, 76.5% KAlF4, 8.5% RbAlF4 and 15% KBF4, on a molar basis, was mixed and fused at 650 DEG  C. for one hour, then cooled slowly until solid.  The product may be used as a dielectric or insulating material, having an electrical conductivity less than 10-6 mhos. per cm.  Specifications 903,657 and 984,896 are referred to.

3,112,207 FLUOALUMINATE COMPOSITION AS DIELECTRIC MATERIAL Isadore Mockrin, Plymouth Meeting, and Alexander Kowalski, Levittown, Pa, assignors to Pennsalt Chemicals Corporation, Philadelphia, Pa., a corporation of Pennsylvania No Drawing. Filed Oct. 11, 1961, Ser. No. 144,308

6 Claims. (Cl. 106-39) and phlogopite, K Mg Al Si O (OH) are the two natural forms of mica used as dielectrics. Synthetic mica, phlogopite in which F has replaced the OH group, is also used to some extent. Such characteristics as high dielectric strength, low power-factor losses, flexibility, trans parency and perfect cleavage make mica usable in many industries.

Sheet mica is a vital material both in peace and Wartime for the electronic and electrical industries. Unfortunately, the United States has been virtually dependent on foreign sources for high-quality sheet mica. India is the principal source of muscovite sheet mica. Brazil is also an important exporter of high-quality muscovite block to the United States, the United States supply of phlogopite is obtained almost entirely from Madagascar and Canada.

Research on synthetic mica, or fluorine-phlogopite, has resulted in a commercially feasible method for manufacture, but only relatively small crystals are generally ob-- tained, so that synthetic mica is not a substitute for sheet mica. However, synthetic mica is used in such appliations as glass-bonded mica ceramics and hot-pressed machinable dielectrics (see for example, the disclosure of US. 2,675,853). Thus, there still remains the need for a readily available sheet dielectric material.

In the application of I. Mockrin, Serial No. 39,726, filed June 29, 1960, issued January 9, 1962, as U.S. 3,016,480, there is described the use of potassium or rubidium tetrafiuoaluminates as dielectric materials. These fluoaluminates may be obtained in a layer structure form providing cleavable sheets similar to natural mica. It has been observed when working with potassium and rubidium tetrafluoaluminates that KAIR; forms less brittle and larger crystalline sheets than does RbAlF On the other hand, RbAlF forms books of crystals which cleave more readily than KAlF Furthermore, it has been observed that at low temperatures (about -23 C.), KAlF undergoes a transition which results in deterioration of the desirable sheet structure.

However, in application Serial No. 102,518, filed on April 12, 1961, by Isadore Mockrin and Alexander Kowalski, it is pointed out that the transition temperature can be greatly lowered and that improved cleavage properties ted States Patent anaztv Patented Nov. 26, 1953 are obtained by providing a composition which is a solid solution of KAlF and RbAlF It has now been found by this invention that improved fluoaluminate dielectric materials are obtained by providing novel compositions which comprise solid solutions of KAlF RbAlF and KBF Such novel compositions have the unexpected propertes of large size crystals and good cleavability and they offer advantages from the standpoint of preparation and ease of fabrication.

The novel compositions of this invention include those mixtures of the ternary solid solution of KAlF RbAlE; and KBR; which contain about 1 to about 15 mol percent of KBF and at least about 4 mol percent of RbAlF These levels of ingredients are significant because below about 4 mol percent of RbAlF the transition temperature of the solid solution is not lowered to the desired level. Expressed another way, it requires at least about 4 mol percent of RbAlE; in the ternary solid solution to achieve the low transition temperature advantages. The upper limit of RbAlR, will be about 90 mol percent since a greater amount in the composition reduces crystal flexibility. A molar proportion of KER; in the range between about 1% and 5% is preferred in order to achieve optimum properties for the ternary solid solution. At levels of below about 1%, the amount of KBE, does not add materially to improved cleavage properties. On the other hand, even when KER; is used in the melt in amounts exceeding-about 15% the ternary product still contains only about 15% because this appears to be the solubility limit of KBR, in the ternary composition. From a prac tical standpoint the ternary composition will preferably contain between about 4% and 50% rubidium tetrafluoaluminate and from about 2% to 5% of potassium fluoborate, the balance being the potassium tetrafluoaluminate.

The novel dielectric compositions of this invention are readily obtained by slowly cooling a melt of the three ingredients, no elaborate controls being required. In this way relatively large sheet-like crystals of the ternary solid solution formed. After cooling the melt, the resulting material contains books of sheet-like crystals of varying size which after removal are split and used in the manner normally employed with mica.

The potassium fiuoborate ingredient of the ternary solid solution is a well known composition prepared from H 80 and HF in the presence of potassium hydroxide according to the method of Borlander, Hollatz, and Fisher (Ben, vol. 65, page 535, 1932).

The fluoaluminate materials (e.g. KAlF and RbAlF are prepared readily according to the method of Brosset- (Z. Anorg. Chem, 235, 139-147 (1937) and 239 301- 304 (1938) which involves simply the evaporation to dryness of a mixture of (1) an aqueous hydrogen fluoride solution of a hydrated alumina (e.g. gibbsite), and (2) the alkali metal (i.e. potassium or rubidium) fluoride, the solid ingredients being used in stoichiometric amounts.

As indicated, these novel fiuoaluminate compositions are solid solutions, by which is meant that the lattice structure of neither constituent is evident in X-ray patterns, but that a composite, single-phase lattice, intermediate between that of the three constituents is observed. The crystalline sheets of this solid solution are transparent, have excellent cleavage properties and in addition show no transition when cooled to temperatures below 78 C. It is quite surprising that KER; exhibits solid solubility in admixture with KAlF and RbA1F The crystal lattice structure of KAlF and RbAlR theoretically requires that any element substituting for aluminum in the lattice network have a coordination number of 6. Boron has a coordination number of 3 or 4, but never 6 and yet it exhibts solid solubility in admixture with KAIE, and RbA1F4- The following examples will serve to further illustrate the invention:

EXAMPLE 1 99.5 parts of potassium tetrafluoaluminate, 131.9 parts of rubidium tetrafiuoaluminate, and 5.0 parts of potassium tetrafluoborate were fused at 650 C. for one hour and then cooled slowly until solid. The matrix contained many books of transparent sheet crystals, some of which were /2 inch by /2 inch. These sheets were cleaved readily to crystals less than one mil thick. The thin crystals were strong and somewhat flexible. The melt composition yielding this product corresponded on a molar basis to 48.6% KAlF 48.6% RbAlF and 2.8% KBF EXAMPLE 2 Following the essential details of Example 1, a melt composition containing on a molar basis 76.5% KbAlF 8.5% RbAlF and 15% KER; was cooled to yield books of transparent sheet crystals easily cleavable to strong, mica-like materials.

EXAMPLE 3 A product similar to that of Example 2 was obtained from a melt composition of 42.5% KAlF 42.5% RbAlE; and 15% K31 EXAMPLE 4.-THERMAL DATA FOR LOW- TEMPERATURE INVERSION The low-temperature inversion was studied by differential thermal analysis (DTA). The furnace and sample holder for this low-temperature DTA consists of a Dewar flask thoroughly chilled with liquid nitrogen prior to the run and a silver sample block on which is wound a resistance winding of insulated wire connected to 110 v. A.C. with a variable voltage transformer. The rate of heating depends on the current applied (up to 1 amp). Cooling curves were run and then heating curves. The former yielded exothermic peaks and the latter endothermic peaks; this is characteristic of inversions.

The following table illustrates the improvement obtained by this invention with respect to the low temperature inversion effect. It is clear from the table that the solid solutions of this invention show the transition at temperatures much below that for KAlF alone and are equivalent in this property to the low inversion temperatures obtained with the binary mixtures of SN. 102,518 referred to above.

EXAMPLE 5 Crystals of various thickness from various molar ratios of KBF -KAlF RbAlF solid solutions were used as the dielectric of a capacitor.

Dielectric measurements were made at 25 C. with a General Radio Capacitance Bridge, Model 716C, for the frequencies 100 c.p.s., 1 kc. and 10 kc. For measuremerit at 100 kc. and higher, a Boonton Q Metcr'was used.

The test sample was a capacitor consisting of two electrodes separated by a solid spacer which spacer was the fluoaluminate sheet. The capacitor was prepared either (a) by vacuum depositing gold or (b) by applying a silver paint to the opposite flat faces of the fluoaluminate sheet and drying at 200 C. for 30 minutes, thus leaving the silver metal as electrodes.

Dielectric strength values were determined on crystal pieces with noncoated or with silver coated samples. The sample in the former case was held between point contacts immersed in an oil bath. The rate of loading, in accordance with an ASTM standard, was step-Wise at 500 volts/minute, cycles until failure. Since the dielectric strength is expressed as volts/unit thickness, the determination of the thickness of the piece investigated is required.

A micrometer measurement of the thickness gives a value which does not reflect localized variations of thickness. Therefore, it was considered more meaningful to first conduct the experiment until failure and subsequently measure the thickness in the immediate vicinity of failure by optical methods. A microscope at a 230 magnification was used. The vertical adjustment of the focal point by a micrometer screw calibrated in microns permits the measurement of thickness in the vicinity of breakdown. The true thickness of the sample is then the linear distance between a focus on the upper and lower crystal surfaces multiplied by the index of refraction. The index of refraction of these materials was assumed to be about 1.4 which is representative of crystalline inorganic fluorine-containing materials.

Both the above methods have been utilized for thickness measurements; however, for the purposes of determination of dielectric strength, the latter optical method is deemed preferable. In general, the test procedures of ASTM designation D 150-54T were followed.

The data given in the following tables represent values obtained in the dielectric tests when using the more physically perfect samples, having a general absence of cracks, striations and other imperfections. Tables II to V give the capacitance (C), dielectric constant (K) and dissipation factor (tan 5x10 for samples of various thickness at various frequencies.

Table II MELT COMPOSITION: 87% KAlF4l0% RbAlF4-3% KBF;

Sample Frequency Tan 8 1O- 0, mil. K

100 c.p.s 150.0 25.90 9.34 l3= 032 mm 1 kc 56. 6 25. 40 9. l5 21. 2 25. 40 9.15 61 83.1 24. 8. 92 112. 2 23. 30 8. 41

LEGEND:

t= thickness of sample. A area of one crystal face. K=dielectric constant.

Table III Table VI.Dielectric Strength MELT COMPOSITION: 77% KA1F420% RbA1F43% KBF4 MELT C OMPOSITION: 87% KAIF41O% RbAlF4-3% KB F Sample Frequency Tan 6X10- O, 1 1f. K Thickness, Total Volts, Dielectric 5 inches X 60 c.p.s. Strength, volts/mil 55. 5 43. 2 10. 70 t=.05 mm 29. 9 42. 8 10. 62 A=22.75 mm 2 15. 5 42. 6 10.57 2. 36 8, 000 3, 390 K=.248 C 63. 7 42. 7 10. 60 2. 50 6, 500 2, 560 106. 5 39. 9 9. 90 2.08 5, 500 2, 640

31. 1 23. 75 8. 83 t= 045 mm 12. 8 23. 45 8. 72 10 2.11 7, 000 3, 320 A=13.64 m 8. 6 23. 35 8. 68 1. 75 4, 000 2, 280 K=.372 C. 32. 5 22. 9O 8. 52 1. 98 6,000 3, 030

2. 26 8, 560 3, 760 2. 59 6, 000 2, 320 Legend: 1. 62 6, 500 4, 010

t=tl1ickness of sample. A=2rea of one crystal face. 3. 52 8,000 2, 270 K=dielectric constant. 3. 99 6, 500 1, 630 2. 54 4, 500 1, 770

Table IV 2. 55 4, 500 1, 765 1. 39 3, 000 2,150 MELT COMPOSITION: 57% KA1F13O% RbA1F43% KBF4 0 1,60 6 500 4 060 2 2 2 Sample Frequency Tan 5X10- C, 1 .11. K 3 730 1 Measured optically [n=1.40 (assumedfl. 53.7 11.15 8.13 2 1 1r ,1 t=.058 m1 21. 5 11.10 s. 11 mm ues A=13.61 mm. 18. O 11 10 8. 11 K: 731 o. 21. 2 10. 5s 7. 72

100 5,115 5 31180 52 Table VII.Dielectric Strength t= .065 mm. 1 110.. 13. 1 36. 7O 7. 49 A 12 10 k 11 7 30 5 7 4 MELT C OMPOSITIONZ 77% KA1F4-20% RbA1F4--3% KBFQ K- 244 C. 11110. 14. 4 30. 41 7. 42

me 85 86 06 30 lhiclmess, Total Volts, Dielectric 10g 11450 89' 60 lnches X 10- 6O c.p.s. Strength, t=.068 mm. 1 kc... c. 02 89. 50 7. 1s volts/11111 !F1:=84.72 mn'lfi 10 kc- 13. 1;

= 0803 C. 1 me. .2

1 1. 54 2,500 1, 522 31110 3 8O 87 52 7 07 1.65 37500 7 0 100 c.p.s. 8. 2 5 35. 5 6.98 56 500 540 t=.1 mm. lkc 5.5 36.3 6.98 A=58.72 mm? 10 kc. 2. 75 3e. 5 e. 92 21500 1, 600 K=.192 0. 1 Inc- 10.52 35. 45 6.80 500 035 3 me. 15. 33. 74 6. 4s 96 5100 550 100 5. 1.5".-. 16.30 43. 0o 7. s4 89 7100 2, 420 t= .035 mm. 1 kc 9. 70 42. 95 7.36 40 63 61000 280 A=23.05 mm. 10 kc. 36. 5 42. so 7. 35 53 7, 960 K 1717 C. 1110.. 3. 4 42. 88 7. 36

1 4 2.86 8, 000 2, 795 31110 5 4 0 89 7 19 3.03 8,000 2,640 2. 56 7, 000 2, 735 Legend:

t=thickness of sample. 37 51000 1 110 A=area of one crystal face. 04 31500 715 K=d1eleotr1c constant. 2 2. 28 290 Table V 1 Measured optically [n=1.40 (assumedfl. MELT COMPOSITION: 57% mum-40% RbAlF.;-3% KBF. 2 Average va s.

Sample Frequency Tan 5 10* C, 1 1f. K V

100 5. 1.5--- 5250 51. 60 10.70 Table VIII .Dielectric Strength 1; 05 111m 1 kc 618 55.20 9. 58

. 76.1 54. 90 9. 54 MELT COMPOSITION: 67% KA1F 30% RbAlF 3% K131 Ih1ckness, Total Volts, Dielectric 21.20 19. 95 8. 62 lnches X 10- c.p.s. Strength, 6. 05 19. 8. 59 volts/mil 6. 02 19. 8. 62 46.20 19. 24 8. 32 59. 50 18. 07 7. 81 l. 57 6, 500 4,140 60 1. 98 7, 000 3, 530 153 40. 40 7. 47 1. 97 6, 500 3, 300

100 c.p.s. 13. 8 36. 35 9. 45 65 2. 23 5, 500 2, 470 t=. 2 111m. 1 kc 9. 3 36. 30 9. 44 2. 48 6, 500 2, 620 A=" .26 1111n. 10 8. 2 36.25 9. 43 1. 57 5, 000 3, 180 K=.26O C. 1 me. 25. 3 34. 98 9. 1O 1 3 mo 59. 4 33. 79 8. 78 1- 01 4, 000 3, 980 2. 58 6, 000 2, 320 2. 42 8, 000 3, 310 Legend:

t=thickness of sample. 7 0 2. 05 7, 500 3, 660 A=2re2 of one crystal lace. 46 500 045 K=dielectric constant. 2. 6O 4, 600 .1, 5 10 e followmg Tables VI and IX 1llustrate the drelectnc strength properties of the compositions of the invention 1 Me s -ed t 11 and Table X 1s for comparatwe value W1thrmuscov1te m1ca. 7 a m on m y [n (assumedn 2 Average values.

Table IX .-Dielectric Strength- IWELT COMPOSITION: 57% KAlF 40% RbAlF -3% KBF Thickness, Total Volts, Dielectric inches X 10- 60 c.p.s. Strength, volts/mil 1 Measured optically [n=l.40 (assumcd)]. 2 Average values.

For comparative purposes Table X is included to show the dielectric strength properties of a commercial mica.

1 Measured optically [n=1.59 (mean value for muscvite)] It is clear that the dielectric strength properties of the compositions of this invention have the same order of magnitude as the commercial mica. It will be evident from the above data that the dielectric strength properties do not appear to vary with the composition of the solid solutions of this invention. However, there does appear to be a tendency toward the same thickness effect reported with muscovite mica in that the greatest dielectric strength is obtained with the thinnest samples. This invention provides for compositions with good cleaving properties and thus enables thin crystal sheets to be readily obtained resulting in dielectric materials of relatively high dielectric strength.

EXAMPLE 6 Observations concerning the effect of composition on ease of cleaving of the crystals are listed in Table XI:

T able'Xl OBSERVATIONS CONCERNING THE CLEAVABILITY OF The samples of the solid solution of potassium and rubidium fiuoaluminates used in the above evaluations were single crystals as obtained from the melts. However, it is also possible to hot press small pieces of the fluoaluminate composition as is done with synthetic mica according to the process of US. 2,675,853.

It will he understood that capacitor-s may be made by techniques other than painting on or vacuum plating metal electrodes. For example, metal foils of aluminum, copper, and other electrically conducting materials may be used as the electrodes which are separated by the dielectric spacer. The fluoaluminate compositions are employed as is mica in conventional manufacturing techniques and U.S. Patents 1,345,754; 1,952,580; and 2,522,713 are illustrative of methods which may be used to make capacitors containing the fluo aluminate dielectric.

In addition to using the novel fluoaluminate compositions of this invention in a capacitor as demonstrated above, they may be used in other electric and electronic applications. For example, they may be used as electrode supports in vacuum tubes, in automated electronic manufactured articles employing wafer dielectrics for micrornodule systems, in motor armature insulation, in high-voltage generator insulation in the form of tape fiabricated from flakes of the fiuoaluminate bonded to a backing, as dielectric insulators in switches, microswitches, relays, transformers, Wires, and cables, coaxial cables, and the like, and in many electrical applications where dielectrics are generally used. In addition, they may be employed in place of glass as a binder for natural and synthetic mica used in pressed dielectrics. In this application they will have the advantage of supplementing the dielectric properties of the mica rather than adversely aifecting the dielectric properties of the mica as does glass.

Many changes will be obvious to the skilled artisan and may be made from the above description of the invention without departing from its spirit and scope.

We claim:

1. Articles of manufacture containing an electrical conductor insulated with a solid dielectric wherein said dielectric is a crystalline solid solution consisting of KAlF RbAlF and KBE, wherein the amount of RbAlE, is at least about 4 mole percent and the amount of KBF is at least about 1 mole percent.

2. Articles of manufacture as in claim 1 wherein the amount of R'bAlF is about 4 to 50 mole percent and the amount of KER, is 2 to 5 mole percent.

3. An electric capacitor comprising a sealed container, a pair of metal electrodes and a solid spacer therebetween, said spacer comprising a crystalline solid ternary solution consisting of about 4 to mole percent of RbA1F 1 to 5 mole percent of KBF and the balance of said ternary solid solution being KAlF 4. A solid solution of a ternary mixture consisting of at least about 4 mole percent of RbAlF l to 15 mole percent of KBF and the balance of said solid solution KBF by cooling a melt containing 67 mole percent of being KA1F KAIF 30 mole percent of R bAlF and 3 mole percent of 5. A crystalline solid solution of a ternary mixture con- K81 sisting of about 4 to 50 mole percent of RbAlR 2 to 5 mole percent of KBF and the balance of said solid solu- 5 References Cited in the file of this Patent tionbeing KAlFi. UNITED STATES PATENTS 6. A process for preparing a crystalline sol1d SOlUtlOll 3:016480 Mockrin Jan 9, 1962 of a ternary mixture consisting of KAIF RbAlR; and 

1. ARTICLES OF MANUFACTURE CONTAINING AN ELECTRICAL CONDUCTOR INSULATED WITH A SOLID DIELECTIRC WHEREIN SAID DIELECTRIC IS A CRYSTALLINE SOLID SOLUTION CONSISTING OF KA1F4, RBA1F4 AND BDF4 WHEREIN THE AMOUNT OF RBA1F4 IS AT LEAST ABOUT 4 MOLE PERCENT AND THE AMOUNT OF KBF4 IS AT LEAST ABOUT 1 MOLE PERCENT. 